1. Technical Field
In general, this invention relates to a specially blended, powdery, larvicidal composition suitable for application to open water catchments to control local mosquito populations, and more especially to the hereinafter described water-free preparation process for making a powder that possesses utility both for abatement of use by mosquitoes of an open water catchment as a successful breeding site, and for conserving water by suppressing the evaporation of water from the same catchment.
A tried-and-proven method of abating the kind of mosquito breeding which is typically localized at a site consisting of an open water catchment is to drain the catchment or procure its drying up, thus depriving egg-laying female mosquitoes of opportunity to use that particular site for breeding. However, whenever conservation of water stored in open catchments is a high priority social objective, a “drying-the-site” method of mosquito control has to be ruled out. In realistic scenarios, therefore, there would be advantage to devising suitable means capable of simultaneously conserving water and controlling mosquitoes. The powder to be manufactured in accordance with the instant invention will constitute such means.
2. Prior Art
The field of mosquito larvicides encompasses a number of variously formulated known toxicological and non-toxicological compositions suitable for application to mosquito breeding waters. Non-toxicological compositions include those known to utilize as a larvicide at least one suitably selected organic compound capable of forming a monomolecular film, or ie., monolayer, on the water catchment surface. The film to be formed in such a case need contain no toxin which kills mosquito larvae, but instead is designed to cause them to sink and drown by so modifying the surface of a catchment that the denser-than-water larvae which normally hang suspended therefrom will be deprived of their usual means of support, viz., surface tension on a ring of hairs surrounding tracheal openings at the upwardly directed, air-snorkeling rear end of each aquatic mosquito larva.
Concerning the field of water conservation, it is wellknown that some although not all organic compounds capable of forming a monolayer on a water surface can be effective as water evaporation suppressants. Since not every kind of monolayer produces a film that is effective for practical water evaporation suppression, it would be factually insupportable to infer that, merely by the formation on mosquito breeding waters of any kind of monolayer whatsoever, that there inherently would result effective water evaporation suppression, always accompanying the intended mosquito control effect. Nor, on the other hand, would it be factually supportable to infer that every formed monolayer that does effectively suppress evaporation of water must inherently be lethal to mosquito larvae.
In 1968, the June issue of Mosquito News published “A FEASIBILITY STUDY ON THE UTILIZATION OF MONOMOLECULAR FILMS FOR MOSQUITO ABATEMENT”, wherein Gerald A. Lorenzen and Wilmon W. Meinke reported, among other things, their finding that film-forming cetyl alcohol, ie., n-hexadecanol, which is wellknown in surface chemistry to be a compound effective for evaporation suppression use, has no larvicidal effect on the Culex quinquefasciatus mosquito species common in Texas.
Lorenzen and Meinke also reported that particles of cetyl alcohol floating on the water surface of test units were observed to be apparently fed upon by mosquito larvae with no ill effect to them. Plausible effect of such feeding behaviour upon the surface area coverage capacity of the amount of finely ground film-forming material used received no mention, but it may reasonably be supposed that an individual cetyl alcohol particle ingested by a mosquito larva does not subsequently participate in usual film-spreading and film-repairing processes associated with use of powdered fatty alcohols for evaporation suppression. A real prospect thus exists that presence of a high population of actively feeding mosquito larvae at a water catchment site can have a deleterious effect on certain evaporation suppression measures, specifically those wherein a fatty alcohol of high enough carbon number to be solid-phase in finely particulate bulk form is utilized. For an example of a film-spreading powder for suppressing water evaporation which contains fatty alcohol particles upon which mosquito larvae may conceivably feed, see U.S. Pat. No. 6,303,133 B1 (O'Brien), incorporated here by reference.
In 1977, the September issue of Mosquito News published “MOSQUITO CONTROL WITH MONOMOLECULAR ORGANIC SURFACE FILMS: I—SELECTION OF OPTIMUM FILM-FORMING AGENTS”, wherein William D. Garrett and Sheldon A. White cited both the abovecited report by Lorenzen and Meinke, and A. S. Msangi's earlier research-based conclusion (1956) that n-hexadecanol produces no appreciable influence on aquatic larvae of the Anopheles gambiae mosquito species common in Africa.
Thus, although a monolayer of cetyl alcohol is known to suppress water evaporation effectively, deficiency of the same film-forming compound for a larvicidal role is also known.
One traceable trend respecting delineation of criteria for optimum materials selection directed to forming monolayers capable of interfering with mosquito breeding diverges in major respects from another traceable trend respecting delineation of criteria for optimum materials selection directed to forming monolayers that effectively suppress evaporation.
Preference for a liquid state for film-forming material in its pre-distribution bulk phase, and for a high fluidity of the formed monolayer on water are two of the several key criteria listed by Garrett and White in the second of the abovecited Mosquito News reports, which from a physico-chemical standpoint are criteria conducive to larvicidal effectiveness. The same two criteria, unfortunately, tend to be incompatible with effective suppression of water evaporation from an open reservoir, particularly if the film-forming material selected is a normally solid-phase fatty alcohol.
According to a classic surface chemistry textbook, INSOLUBLE MONOLAYERS AT LIQUID-GAS INTERFACES by George L. Gaines, Jr. (Interscience Publishers, 1966), high fluidity as a formed monolayer property runs counter to optimum effectiveness at suppressing evaporation. The kind of monolayer that imposes higher resistance (than other kinds) to egress through it of water vapor is the so-called “condensed film” kind of monolayer, featuring close side-by-side packing of upright unbranched molecular chains. Extent of vapor passage resistance increases with chain length, provided the chain is truly linear so that adjacent molecules in a film can be closely packed.
Fatty alcohols with a carbon number lower than 14 are liquid at standard temperature, and even if unbranched form a film of high fluidity on water, rather than forming the condensed kind of monomolecular film that retards evaporation significantly more effectively. Thus, lauryl alcohol, ie., dodecanol, with 12 carbon atoms per molecule, would not be recommended for optimization of evaporation suppression properties, although it would be selectable for its ability to cause mosquito larvae mortality in the manner reported by Lorenzen and Meinke in the first of the abovecited Mosquito News reports.
The discussions of larvicidal agencies present in both abovecited Mosquito News reports are predicated on the concept that mosquito breeding at a given open water site can feasibly be abated on a non-toxicological basis by suitably modifying the water surface to prevent normal larval respiration.
Another relevant factor in the background of the instant invention is the known use of biological mosquito larvicides comprising toxin-carrying spores of microbial species, eg., Bacillus thuringiensis variety Israelensis, and Bacillus sphaericus, which shall collectively be referred to hereinafter as ‘mosquito-specific entomopathogens’. The toxins are specifically lethal to larvae of mosquitoes when the minute Bacillus spores are ingested thereby.
It has also been suggested that a so-called “synergistic” mode of treatment of a mosquito breeding site can be practiced, wherein larvicidal agencies include both the surface tension reduction by a monolayer that drowns mosquito larvae, and the use of mosquito-specific entomopathogens to poison them. Film-forming compounds recommended for known compositions for carrying out such a “drowning-plus-poisoning” type of larvicidal treatment are compatible with the optimized materials selection criteria proposed in the abovecited Garrett and White Mosquito News report, because these are typically branched chain compounds that procure the highly fluid—rather than condensed—kind of monolayer. For a citable example wherein ethoxylated and branched-chain compounds are recommended see the “Insecticide composition for controlling insects which have an aquatic breeding site” by A. I. McMullen, U.S. Pat. No. 4,707,359 (Nov. 17, 1987). The composition by McMullen is not designed to procure effective evaporation suppression.
Significantly, concerning preparation of formulations according to the McMullen insecticidal monolayer-forming invention, problems that would be encountered if attempting to evenly distribute mosquito-specific entomopathogens within a main body comprising film-forming constituents were easily avoided by the use of water as a dispersion medium during product preparation. Contrastingly, the film-spreading powder of abovecited U.S. Pat. No. 6,303,133 B1 (O'Brien) does not lend itself during its preparation process to admixture of its constituents with water, because addition of water during processing would prematurely activate ionization of the slaked lime, ie., calcium hydroxide, that is included to provide a unique dispersal mechanism that takes effect when the powder is broadcast upon a body of water intended to be coated by an effective evaporation-suppressing monolayer. Without feasibility of using water in the processing, a real problem of how to evenly distribute mosquito-specific entomopathogens within a main body comprising dry particulate film-forming and other constituents arises.
It has already been noted above that the O'Brien patented powder, so long as it places finely divided alcohol particles on a water catchment surface, is susceptible to a number of such particles being ingested by mosquito larvae, if present, thus suffering a loss of reserve film-forming material that would be expected to be of a magnitude proportionate to the number of feeding larvae. Insofar as is known at this time of disclosing the instant invention, nobody has previously suggested the specific manner of resolving this problem that is presently proposed.
As a final point closing background discussion, it is acknowledged as having already been known in the general art pertaining to insecticides to disperse mosquito-specific entomopathogens in a quantity of liquid-phase lauryl alcohol, ie., dodecanol, which is not a satisfactory evaporation suppressant material, although as far back as 1968 (Lorenzen and Meinke) this 12-carbon fatty alcohol was proposed to be suitable for forming a monomolecular film intended specifically for mosquito abatement use.